The experiments proposed in this AREA application focus on regionalization of the anterior neural plate. Work from a previous award defined the period of anterior neural plate determination. The present study builds on these results by examining the expression of early markers for specific forebrain regions, including markers of the telencephalon, the eye, and the pituitary, as well as other regions to assess whether retinoic acid influences forebrain patterning prior to, or after the expression of region-specific markers. The proposal describes a series of carefully designed experiments utilizing in vitro and in vivo methods to assess the role of the prechordal plate on regionalization of the forebrain including transplantation extirpation and co-culture.